The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.
The present invention relates to methods and apparatuses for degaussing, more particularly to methods and apparatuses for degaussing AC electrical equipment such as may be used aboard marine vessels.
There is a trend in the maritime industry toward electric propulsion. The U.S. Navy anticipates the conversion of some of its ships to xe2x80x9call-electricxe2x80x9d ships. For example, large, high-power electric generators and motors will be used onboard to propel the naval vessels. High-current motors and generators can generate large magnetic fields at extremely low frequencies including 60 Hz and harmonics (power frequencies), some of which may leak out of the machine or system and into the surrounding water. These fields can be availed of by underwater mines, torpedoes and surveillance systems for detecting the presence of the vessel, and/or for detonation of explosives. In addition, the leakage fields may interfere with important shipboard systems.
One proper approach to reducing the leakage fields (stray fields) from high-power shipboard devices would be to design the systems according to previously established guidelines. See, e.g., xe2x80x9cDesign of Electrical Equipment with Small Stray Magnetic Fieldsxe2x80x9d, Military Handbook, MIL-HDBK-802 (SH), Jul. 2, 1990, incorporated herein by reference. However, technical and cost constraints may prevent the stray fields from being reduced down to acceptable amplitudes. To further reduce the power frequency electromagnetic emanations from onboard electrical systems will require active cancellation of the stray fields.
Degaussing systemsxe2x80x94including, more specifically, Closed-Loop Degaussing (CLDG) systemsxe2x80x94have been designed to automatically monitor and compensate the static magnetic field signatures of ships produced by the ferromagnetic material used in its construction. See, e.g., R. A. Wingo, J. J. Holmes, and M. H. Lackey, xe2x80x9cTest Of Closed-Loop Degaussing Algorithm On A Minesweeper Engine,xe2x80x9d Proceedings of 1992 ASNE Conference, May, 1992, incorporated herein by reference. See also U.S. Pat. No. 5,189,590 to Carl S. Schneider issued February 23, 1993 entitled xe2x80x9cClosed-Loop Multi-Sensor Control System and Method,xe2x80x9d incorporated herein by reference. The CLDG system is comprised of many static magnetic field sensors (magnetometers), placed throughout the ship, which measure the fields at specific points and then transmit their data to a central computer. Using a special signature prediction algorithm, the CLDG controller computes required changes in degaussing coil currents that will re-optimize the vessel""s signature when changes in its residual magnetization have occurred.
A typical CLDG system is very limited in bandwidth as well as in spatial fidelity for purposes of controlling and canceling magnetic fields. The CLDG system""s sensors, data communication network, controlling computer and degaussing coil design are not even remotely characterized by that which would be required for a power frequency electromagnetic field cancellation system, viz., a wideband digital network comprising a high speed computer along with a large number of wideband sensors and compensating coils. Implementation of this kind of digital network would be impractical in shipboard applications.
In view of the foregoing, it is an object of the present invention to provide method and apparatus for actively reducing or canceling the extremely low frequency (including but not limited to 60 Hz and harmonics) electromagnetic fields generated by shipboard electrical and electromechanical systems (e.g., electric motors and generators).
It is a further object of the present invention to provide such method and apparatus which are practical and economical.
The present invention obviates the need for a wideband digital network including a high-speed computer. Instead, the present invention features the use of small, localized sensing and compensating coils, and further features analogue control thereof
In accordance with many embodiments of the present invention, degaussing apparatus comprises at least four coil units for bordering upon an electrical device so that the respective imaginary planes defined by the coil units together form an imaginary enclosure for the device. Each coil unit includes plural conductors so that at least one conductor is adaptable to connection with a first amplifier for sensing the magnetic flux associated with the device and producing a voltage proportional to the magnetic flux, and so that at least one other conductor is adaptable to connection with a second amplifier for producing a current which neutralizes the magnetic flux. The total or overall effect of these neutralizations of the magnetic flux is to prevent the escape of any magnetic flux from the imaginary enclosure.
A typical embodiment of a power frequency electromagnetic field compensation system in accordance with the present invention comprises an active feedback control system which cancels the electromagnetic field radiating from electromechanical devices (electric motors, generators, control systems, distribution systems, etc.) at a given frequency (e.g., a frequency of 60 Hz), and which also cancels the harmonics of the electromagnetic field. According to frequent inventive practice, the source of the power frequency electromagnetic field (e.g., a motor, a generator, etc.) is completely surrounded by a minimum of three pairs of coils, one pair in each orthogonal direction, wherein each coil comprises at least one multi-conductor cable. In each cable, a first half of the conductors are connected in series, and a second half of the conductors are separately connected in series, thereby forming two independent circuits. The first circuit acts as an induction sensor (sensing coil), the output of which is proportional to the rate-of-change in the magnetic field, while the second circuit acts as a compensation coil to cancel the AC flux passing through the loop formed by the cable. An analogue feedback electronics device drives the second circuit with current so as to force the output voltage of the first circuit to zero. In this way, the total power frequency electromagnetic field emanating from the electromechanical device is cancelled.
The present invention affords several advantages and new features vis-axe2x80x2-vis what is presently achieved or achievable with the static magnetic Closed-Loop Degaussing (CLDG) system. Firstly, unlike the CLDG system, state-vectors for the power frequency electromagnetic field compensation system do not have to be measured or computed according to the present invention. Establishing CLDG state-vectors empirically is a time-consuming and labor-intensive process.
In addition, CLDG systems have to be re-calibrated periodically. In contrast, according to the present invention, once the gains of the amplifiers in the power frequency electromagnetic field compensation system are set (e.g., through the use of a controller), no further calibration or re-calibration is required.
Moreover, a conventional CLDG system requires state-vectors for virtually every circuit configuration and current distribution that could be found inside the electromechanical system being compensated. By comparison, the present invention""s power frequency electromagnetic field compensation system does not need any state-vectors. Therefore, the inventive compensation system can automatically compensate for any changes in current distribution that may occur inside its controlled volume.
Furthermore, generally speaking, it would be difficult or impractical to endow a CLDG system with power frequency electromagnetic field compensation capabilities. Factors such as the bandwidth, sensor dynamic range, number of data channels and update rate of a kind of power frequency electromagnetic field compensation system which could be associated in theory with a CLDG system would be too demanding for the digital data acquisition, transmission and compensation control system characterizing the CLDG system. In accordance with the present invention, a plural number of small, localized, collocated, analogue sensing and compensating coils avoids the drawbacks of a wideband digital network and high-speed computer. Although it would not be impossible to construct a digital power frequency electromagnetic field compensation system in association with a CLDG system, it is believed that the present invention""s distributed analogue system will generally prove to be more reliable and cost effective.
Other objects, advantages and features of this invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.